As known in a number of technical applications, the deflection of a collimated light-beam can be controlled by an electrical signal so as to illuminate preferential regions, as in printing machines or fixed-image reading machines, to modulate or switch optical signals, such as in equipment for optical fiber tranmission systems, to process signals in scientific apparatus, etc.
In the domain of digital telecommunications using light signals, particular performances in terms of deflection velocity are further required, e.g. in the implementation of switching circuits for high bit rate systems. The beam collimated at the input is to be kept, as far as possible, collimated at the output after deflection to avoid drawbacks, e.g., to avoid coupling losses with optical fibers whenever the beam is focused with an angle larger than the acceptance angle.
Finally, a deflection angle as large as possible is required especially when scanning large surfaces.
Nowadays, the most-widely used method of deflecting a light beam uses mirrors oriented by electrical motors. In case a continuous scanning is desired, mirrors are placed along the faces of a rotating prism. This method, however, does not always prove satisfactory because of the limited scanning frequency, which can be at the utmost a value of the order of some KHz, of mechanical complexity and of mechanical part wear, even though very large scanning angles are obtainable.
A more sophisticated technique handles an optical beam with acousto-optic means, (see page 517 of the book entitled "Acoustic waves", written by Gordon S. Kino, Prentice-Hall). In this case, a plane acoustic wave is launched into a crystal to cause a periodic structure of rarefaction and compression regions in the material. As a consequence a sequence of higher refractive-index zones alternated with lower-refractive index zones, wholly equivalent to a diffraction grating, is obtained. The light beam is launched into the crystal at a convenient angle with respect to the grating lines and at the output a reflected beam is obtained with an angle dependent on the acoustic signal wavelength. However, the deflection angles obtained are rather small. Typically they are of the order of a few degrees.
Devices operating on the basis of the electrooptical properties of some materials, such as lithium niobate, are also known. The article entitled "Electrooptic Fresnel lens-scanner with an array of channel waveguides," Applied Optics, Vol. 22, No. 16, 15 Aug. 1983, describes a Fresnel lens, obtained by deposition on a lithium niobate plate of conveniently-spaced electrodes alternately connected to two conductors. Refractive-index variations are obtained by applying a convenient potential difference, that is why the emerging beam, covering equivalent optical paths of different length, undergoes different phase variations. As a consequence, at the output regions the optical beams is focussed when the interference is constructive. Hence it is a lens.
Further electrooptic devices, e.g. as described in the U.S. Pat. No. 4,415,226, have a plate of electrooptic material with electrodes separetely fed and connected to a control apparatus. In this case a Bragg grating is implemented, wherein the deflection angle of the outgoing beam a function of the input angle and of the grating spacing, which cannot be modified once the device has been fabricated, hence only two directions of the outgoing optical beam are possible.